Avalanche forecast

Flathead Range & Glacier NP

Flathead Avalanche Center
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Avalanche danger

Thu
Alpine
2 · Moderate
Treeline
2 · Moderate
Below
1 · Low
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Highlights

Wind-sheltered slopes less than 35 degrees are today's go-to. You can enjoy great riding while minimizing your chances of triggering large avalanches that break on buried weak layers or small to large slabs of wind-drifted snow. Bust out that inclinometer - a digital one on your phone or an analog version - and measure slope angles; misjudging is easy but can carry costly consequences. 

Avalanche problems

  • Persistent Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–3

    Mind your slope angles today, so you can enjoy the great riding while minimizing your risk of triggering slabs that break 15 to 30 inches deep. These slabs are sitting on weak, faceted snow that collapses easily, as evidenced by the loud whumpfs reported in the past week. The slabs are most likely to release on slopes steeper than about 35 degrees. Build a buffer against surprises by riding on slopes that are closer to 30 degrees. Steer clear of likely trigger points like convex portions of slopes and thin, wind-scoured spots. 

    (12/6/2022)

    An 18" thick crown of a soft slab avalanche east of WMR. 

    Photo: Mark Dundas

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2

    Gusty winds have drifted new and recent into dense slabs that you can trigger on steep slopes. These are most widespread at upper elevations and in areas favored by Tuesday's snow, such as John Stevens Canyon and the Swan Range. Slabs large enough to hurt or bury a person will be less common at mid-elevations. The most straightforward way of dealing with this hazard is to avoid terrain that collects wind-blown snow: the lee sides of ridges and crossloaded gullies and slopes downwind of saddles and passes. Debris from triggered slabs of drifted snow can release slabs that break deeper in the snowpack, making even small slides consequential.

    (12/4/2022)

    Scouring, wind-hardened surface, and thin, hard slabs near ridgelines. Swan Range.

    Photo: FAC

Avalanche Discussion

Let's focus on the effects of the winds Tuesday and today, even though our most serious hazard remains triggering slabs that break on buried persistent weak layers. That's because the winds have created a new hazard and may exacerbate the Persistent Slab avalanche problem. 

Wind speeds picked up Tuesday morning, with ridgeline weather stations recording hourly averages in the mid to upper teens. That's right about the threshold where winds start drifting snow, and indeed, a party in the Flathead Range reported drifting snow partially filling their uptrack between laps. Sarah and Mark saw only light drifting in the southern Whitefish Range. Both parties described some cracking in the drifted snow. These reports are from mid-elevation terrain, so I'm guessing that drifting like this was more extensive at upper elevations, with slabs 1 to 2 feet forming in terrain that collects wind-blown snow. With winds continuing today, I expect these slabs to continue building slowly. They can break wide enough and deep enough to bury or injure a person, so it's wise to avoid them, especially when there's plenty of great riding in softer snow on wind-sheltered slopes. If you get into upper elevation terrain today, let us know if you found these slabs.

Now for the "exacerbate the Persistent Slab avalanche problem" part. The complications come in two main ways. The first is that where the freshly-formed wind slabs have formed on slopes riddled with the November weak layers, the increased load can make initiating slides easier. And the size of triggered slides, larger. So a small wind slab can have disproportionately serious consequences. 

The second complication is more subtle. Even small amounts of wind-drifted snow can stiffen the surface of a slab, making it easier to trigger. How much drifting snow? How stiff? Unfortunately, snow science hasn't given us the tools to answer those questions yet. But it's relatively easy to determine when the surface of a slab has been stiffened by the wind. The texture looks different - a little dappled or scalloped, with maybe some conifer needles on the surface. And it'll feel a little thicker and denser as you move through it. 

More importantly, you can have your fill of great riding with little exposure to Wind or Persistent slab avalanche hazards, by sticking to wind-sheltered slopes less than 35 degrees. You can increase your margin of safety by sticking to slopes closer to 30 degrees that don't have steeper convexities. It's easy to underestimate slope angles, so use an inclinometer. And a modified version of that carpenter's adage: measure twice, ride again. Slope angle, slope angle... that's going to be the refrain for a while until the November facets strengthen.

Education - Our next Avalanche Awareness presentation is tonight, December 7, at Whistling Andy Distillery, Bigfork. You can get more details about this free event here. Next week, we will offer an Avalanche Awareness presentation Wednesday, December 14, at Penco Power Products, Kalispell. You can get more details about this free event here.

Observations - Thanks to everyone who's been utilizing our new observation submission page. Try it out and help your community by sharing what you see in the backcountry. The form requires you to fill out the location fields and put a dot on a map. Be as vague or specific as you want. Our new Observation View page highlights signs of instability and allows you to filter by date or zone to pull out reports relevant to your objectives. 

Preseason restrictions on uphill travel are in effect at Whitefish Mountain Resort.

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